This research study will be looking at elderly persons who suffer memory loss and healthy elderly persons. The purpose of this study is to compare two kinds of instruments that measure sleep. One instrument is called wrist actigraphy and the other is polysomnography (PSG). PSG is considered the standard in sleep detection. For this reason it will be performed simultaneously with wrist actigraphy in order to determine how well wrist actigraphy detects sleep. We will be testing these devices on both healthy people and people suffering memory loss. This is because we not only want to know how accurate wrist actigraphy is, but we want to compare how well wrist actigraphy detects sleep in healthy elderly people with how well it detects sleep in those with memory loss.